According to a recent theory, Archean-age gold-quartz vein systems were formed over two billion years ago from magmatic fluids that originated from molten granitelike bodies deep beneath the surface of the Earth. This theory is (5) contrary to the widely held view that the systems were deposited from metamorphic fluids, that is, from fluids that formed during the dehydration of wet sedimentary rocks. he recently developed theory has considerable practical importance. Most of the gold deposits discovered during(10) the original gold rushes were exposed at the Earth’s surface and were found because they had shed trails of alluvial gold that were easily traced by simple prospecting methods.
Although these same methods still lead to an occasional discovery, most deposits not yet discovered have gone(15) undetected because they are buried and have no surfaceexpression.
The challenge in exploration is therefore to unravel the subsurface geology of an area and pinpoint the position of buried minerals. Methods widely used today include(20) analysis of aerial images that yield a broad geological overview; geophysical techniques that provide data on the magnetic, electrical, and mineralogical properties of the rocks being investigated; and sensitive chemical tests that are able to detect the subtle chemical halos that often(25) envelop mineralization. However, none of these high technology methods are of any value if the sites to which they are applied have never mineralized, and to maximize the chances of discovery the explorer must therefore pay particular attention to selecting the ground formations most(30) likely to be mineralized. Such ground selection relies to varying de美国GREes on conceptual models, which take into account theoretical studies of relevant factors.
These models are constructed primarily from empirical observations of known mineral deposits and from theories35)of ore-forming processes. The explorer uses the models to identify those geological features that are critical to theformation of the mineralization being modeled, and then tries to select areas for exploration that exhibit as many of the critical features as possible.
1. The author is primarily concerned with
(A) advocating a return to an older methodology
(B) explaining the importance of a recent theory
(C) enumerating differences between two widely used methods
(D) describing events leading to a discovery
(E) challenging the assumptions on which a theory is based
2. According to the passage, the widely held view of Archean- age gold-quartz vein systems is that such systems
(A) were formed from metamorphic fluids
(B) originated in molten granitelike bodies
(C) were formed from alluvial deposits
(D) generally have surface expression
(E) are not discoverable through chemical tests
3. The passage implies that which of the following steps would be the first performed by explorers who wish to maximize their chances of discovering gold?英语作文
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